The Occurrences Of Ca2uo2(Co3)(3) Complex In Fe(Ii) Containing Deep Groundwater At Forsmark, Eastern Sweden

Elevated U concentrations, most evident in a section similar to 500 mbsl, have been measured in deep Fe(II)-containing groundwater at Forsmark, eastern Sweden and have prompted detailed geochemical and isotopic investigations. The highest U contents (up to 175 mu g/L) are associated with HCO3- of 120-135 mg/L and Ca2+ of 900-1050 mg/L. Geochemical modelling shows that elevated dissolved U can be stabilized by Ca-uranyl-carbonate complexes. Indeed, time resolved luminescence spectrometry confirmed the Ca2UO2(CO3)(3)(0) complex, which is identified in deep reducing groundwater for the first time. U isotopes have been monitored in several sections with high U, and show stable but fracture specific activity ratios (ARs) around 1.5 to 2, although the U concentration varies. This is explained by mobilization of a solid phase with the same AR present in the fracture system close to the sampled sections. The AR > 1 in this solid phase indicates a Quaternary age. (C) 2017 The Authors. Published by Elsevier B.V.